Previously known injection devices are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and a trigger that, when activated, causes the drive spring to act on the syringe when a releasable locking mechanism is also engaged.
Generally, the trigger is rotatable about an axis so that when it is depressed at a first end, a second end (which normally engages the drive spring) is also rotated, thereby releasing the drive spring, extending the syringe and discharging its contents. The trigger comprises a protrusion which is engageable with a cut-out on the releasable locking mechanism when the releasable locking mechanism is engaged, thereby allowing the trigger to be activated. When the releasable locking mechanism is not engaged, the protrusion abuts a portion of the releasable locking mechanism preventing rotation of the trigger and release of the drive spring. This way, accidental activation of the trigger can be prevented.
A problem with an injection device of this type is that the protrusion on the trigger flexes when a force is applied to the trigger and the releasable locking mechanism is not engaged. A strong force applied to the trigger can cause enough flex in the protrusion that the end of the protrusion can engage the cut-out on the releasable locking mechanism, thereby allowing the trigger to be activated even when the releasable locking mechanism has not been engaged.